The present invention relates to a printing press, and in particular the present invention relates to an offset printing press in which a plate cylinder is mounted for skew and throw off movement relative to a cooperating blanket cylinder.
A conventional offset printing press includes two printing couples. Each printing couple includes a plate cylinder and a blanket cylinder. The plate cylinder is disposed in rolling engagement with the blanket cylinder and transfers an ink impression onto the blanket cylinder. The blanket cylinder in turn transfers the ink to the material being printed. The second printing couple includes a second plate cylinder and a second blanket cylinder which prints on a second side of the material at the same time that the first blanket cylinder prints on a first side of the material. This type of press is known as a perfecting press.
It has been common practice to mount at least one cylinder of each printing couple of a perfecting press in eccentrics for the purpose of throw-off and throw-on of the cylinders relative to each other. Also, it is common practice to mount a plate cylinder in a skew eccentric for skewing the plate cylinder relative to the blanket cylinder. Also, in an offset perfecting press it is known to mount a plate cylinder in eccentrics to skew the cylinder relative to a blanket cylinder and to throw off the plate cylinder. See U.S. Pat. No. 3,633,503.
In the prior art when the plate cylinder is skewed, the skew eccentric is turned causing the axis of rotation of the plate cylinder to skew with respect to the blanket cylinder axis. When skewing occurs the distance between the centers of the plate and blanket cylinders changes. This can result in an undesired change in the printing pressure, particularly if a relatively large amount of skew is desired.